The present disclosure relates to a fuel cell and a method for manufacturing the same, an electronic apparatus, an enzyme-immobilized electrode and a method for manufacturing the same, a water repellent agent, and an enzyme-immobilizing material. In particular, the present disclosure is suitably applied to a biofuel cell that uses an enzyme and various electronic apparatuses that use the biofuel cell as a power source.
Fuel cells have a structure in which the cathode (oxidizer electrode) and the anode (fuel electrode) face each other with an electrolyte (proton conductor) therebetween. In conventional fuel cells, the fuel (hydrogen) supplied to the anode is oxidized and separated into electrons and protons (H+); the electrons are delivered to the anode; and H+ moves through the electrolyte to the cathode. At the cathode, the H+ reacts with oxygen supplied from the outside and electrons transmitted from the anode through an external circuit to generate water (H2O).
As described above, fuel cells are highly efficient power-generating devices that convert the chemical energy possessed by a fuel directly into electrical energy, and are capable of extracting the chemical energy possessed by fossil energy, such as natural gas, petroleum, or coal, as electrical energy with high conversion efficiency regardless of the place of use or time of use. Therefore, conventionally, research and development have been actively carried out on fuel cells for application to large-scale power generation, etc. For example, it has been proved that fuel cells installed in space shuttles are capable of supplying electrical power as well as water for the crew and that fuel cells are clean power-generating devices.
Furthermore, in recent years, fuel cells, such as solid polymer fuel cells, that have a relatively low operating temperature range from room temperature to about 90° C., have been developed and have been receiving attention. Therefore, not only application to large-scale power generation, but also application to small systems such as power sources for running automobiles and portable power sources for personal computers and mobile devices has been sought after.
As described above, fuel cells are believed to have a wide range of applications from large-scale power generation to small-scale power generation, and have been receiving much attention as highly efficient power-generating devices. However, in fuel cells, natural gas, petroleum, coal, or the like is normally converted into hydrogen gas using a reformer, and the hydrogen gas is used as a fuel, which poses various problems in that limited resources are consumed and fuel cells need to be heated to high temperature and require a catalyst composed of an expensive noble metal such as platinum (Pt). Furthermore, even in the case where hydrogen gas or methanol is directly used as a fuel, the handling thereof requires care.
Under these circumstances, focusing on the fact that the biological metabolism that takes place in living things is a highly efficient energy conversion mechanism, the application of biological metabolism to a fuel cell has been proposed. Herein, biological metabolism includes respiration, photosynthesis, and the like taking place in microorganism cells. Biological metabolism has a characteristic in that its power generation efficiency is very high and the reaction proceeds under mild conditions such as at about room temperature.
For example, respiration is a mechanism with which nutrients such as saccharides, fats, and proteins are taken into microorganisms or cells; the chemical energy thereof is converted into oxidation-reduction energy, i.e., electrical energy by reducing nicotinamide adenine dinucleotide (NAD+) to reduced nicotinamide adenine dinucleotide (NADH) in the process of generating carbon dioxide (CO2) through a glycolytic pathway and a citric acid (TCA) cycle including many enzyme reaction steps; and in an electron transport system, the electrical energy of the NADH is directly converted into the electrical energy of a proton gradient, and also oxygen is reduced to generate water. The electrical energy obtained here generates, through an adenosine triphosphate (ATP) synthase, ATP from adenosine diphosphate (ADP). The ATP is used for reactions required for the growth of microorganisms and cells. Such energy conversion takes place in cytosol and mitochondria.
Furthermore, photosynthesis is a mechanism with which, in the process of taking in light energy and converting the light energy into electrical energy by reducing nicotinamide adenine dinucleotide phosphate (NADP+) to reduced nicotinamide adenine dinucleotide phosphate (NADPH) through an electron transport system, water is oxidized to generate oxygen. The electrical energy is used for a carbon immobilization reaction in which CO2 is taken in and for synthesis of carbohydrates.
As a technology in which the biological metabolism described above is used for a fuel cell, a microbial cell has been reported, in which electrical energy generated in microorganisms is taken out of the microorganisms through an electron mediator and the electrons are delivered to an electrode to obtain an electric current (e.g., refer to PTL 1).
However, microorganisms and cells include many unnecessary reactions other than target reactions that convert chemical energy into electrical energy. Thus, in the above-described method, electrical energy is consumed in undesired reactions, and sufficient energy conversion efficiency is not achieved.
Under these circumstances, fuel cells (biofuel cells) in which only a desired reaction is carried out using an enzyme have been proposed (e.g., refer to PTLs 2 to 13). In such biofuel cells, a fuel is decomposed by an enzyme and separated into protons and electrons. There have been developed biofuel cells that use, as a fuel, alcohols such as methanol and ethanol or monosaccharides such as glucose.    PTL 1: Japanese Unexamined Patent Application Publication No. 2000-133297    PTL 2: Japanese Unexamined Patent Application Publication No. 2003-282124    PTL 3: Japanese Unexamined Patent Application Publication No. 2004-71559    PTL 4: Japanese Unexamined Patent Application Publication No. 2005-13210    PTL 5: Japanese Unexamined Patent Application Publication No. 2005-310613    PTL 6: Japanese Unexamined Patent Application Publication No. 2006-24555    PTL 7: Japanese Unexamined Patent Application Publication No. 2006-49215    PTL 8: Japanese Unexamined Patent Application Publication No. 2006-93090    PTL 9: Japanese Unexamined Patent Application Publication No. 2006-127957    PTL 10: Japanese Unexamined Patent Application Publication No. 2006    PTL 11: Japanese Unexamined Patent Application Publication No. 2007-12281    PTL 12: Japanese Unexamined Patent Application Publication No. 2007-35437    PTL 13: Japanese Unexamined Patent Application Publication No. 2007-87627